1. Field of the Invention
The present invention relates to the technology of organic emitting materials, and particularly to the novel phosphorescent emitters, wherein the said phosphorescent emitters belong to a class of iridium (III) based phosphor bearing a pincer carbene chelate.
2. Description of Related Art
It is well known that organic light emitting diode (OLED) device was initially invented and proposed by Eastman Kodak through a vacuum evaporation method. Tang and VanSlyke from Kodak deposited a multilayer architecture of organic semiconducting materials, such as aromatic diamine and Alq3 on a transparent indium tin oxide (abbreviated as ITO) conducting glass to form the hole transporting layer (HTL), light emitting layer (EML), and subsequently completed the fabrication of an organic electroluminescent (EL) device by depositing a metal electrode on top of the Alq3 layer. Thereafter, the respective EL devices become the new generation of lighting device for flat panel displays or solid state luminaires because of the high brightness, fast response time, light weight, compactness, true color, wide viewing angles, without the need for LCD backlighting plates, and low power consumption.
One important factor that controlled the luminescence efficiency of OLEDs is the light-emitting material within the emitting layer. It has been proposed that the emission is produced from the excitons derived from the recombination of electrons and holes in the light-emitting layer (material) of OLED devices. According to electron spin statistics, the ratio of the triplet excitons versus the singlet excitons is approx. 3:1 upon conducting the electric excitation. So that, when a fluorescent material is used as the light-emitting layer of OLED, only the 25% of the singlet excitons can be used to generate the luminescence, while the rest of 75% of triplet excitons are lost through the non-radiative processes. For this reason, the typical fluorescent material would produce a maximum internal quantum efficiency of 25%, which amounts to an external quantum efficiency of only 5% in the as-fabricated OLED devices.
Cyclometalated Ir(III) metal complexes, such as red emitting Ir(btp)2(acac) (i.e., bis(2-(2′-benzothienyl)-pyridinato-N,C3′)iridium(acetylacetonate)), green emitting Ir(ppy)3 (i.e., fac-tris(2-phenylpyridine)iridium(III)), and blue emitting Firpic (i.e., bis[2-(4,6-difluorophenyl)pyridinato-C2,N](picolinato)iridium(III)]), belong to a class of distinctive phosphorescent emitters. The chemical structures of the aforementioned Ir(btp)2(acac), Ir(ppy)3 and Firpic are represented by the chemical formulas I′, II′ and III′ showed below:

In 2006, Andrew et al. reported a research paper entitled “Luminescent Complexes of Iridium (III) Containing N^C^N-Coordinating Tridentate Ligands”, in which an iridium (III) metal complex bearing tridentate N^C^N-coordinating chelate has been proposed to be a potentially useful phosphorescent emitter for fabrication of OLEDs and associated optoelectronic devices. These Ir(III) metal complexes are named as Ir(dpyx)(dppy) and Ir(dpyx)(F4dppy), wherein their chemical structures are represented by following chemical formulas IV′ and V′, respectively.

However, these cyclometalated Ir(III) metal complexes reveal three important drawbacks when used as emitters in fabrication of OLEDs: e.g. poor luminescence efficiency, non-tunable color hue, and low synthetic yield (37% and 21%, respectively). Therefore, the person skilled in OLED art is able to assume that, these cyclometalated Ir(III) metal complexes cannot be produced in larger quantity because of the higher manufacturing cost, and cannot be served as the suitable light-emitting material due to the practical difficulty in conducting the color tuning. Moreover, since the pyridine ligand in the aforesaid cyclometalated Ir(III) metal complex linked to the central metal atom through two terminal Ir—N coordination bonds, the associated bond energy is not strong enough to induce a sufficiently large crystal field for destabilizing the metal-centered dd excited state, which usually served as the quenching state that can effectively reduce the emission quantum yield under all conditions. For this reason, the cyclometalated Ir(III) metal complex cannot afford suitable stability and luminescence efficiency.
Moreover, it is notable that, the emitters at the lowest energy excited states in an organic light-emitting material are capable to be promoted to the higher lying metal-centered dd excited state by thermal population. As a result, the excitons at the metal-centered dd excited state may possess longer emission lifetime and have higher tendency for undergoing non-radiative deactivation, resulting in a significantly reduced emission quantum yield. Such an observation is particularly notable for the typical blue or true-blue emitting phosphorescent materials.
Therefore, according to above descriptions, the person skilled in the art of OLED fabrication and material design are able to deduce that the conventional cyclometalated Ir(III) metal complexes and/or the common blue-emitting materials have the following drawbacks and shortcoming: (1) the crystal field of the iridium (III) metal complex is not strong enough to warrant a relatively higher lying metal centered dd excited state; (2) the physical stability of the iridium (III) metal complex is inadequate; (3) the non-radiative decay in the blue-emitting material cannot be effectively reduced; and (4) the quantum yield of the blue-emitting material is too low to have any practical applicability.
Accordingly, in view of the conventional cyclometalated Ir(III) metal complexes and the commercial blue-emitting materials still possess many drawbacks, the inventor of the present application has made great efforts on research thereon and eventually provided a series of iridium (III) based phosphors bearing a pincer carbene chelate, which are novel phosphorescent emitters capable of serving as excellent dopant emitters in OLEDs.